He probably was safe
There are two instances here that I think are of crucial importance, both from TNG 'Datalore'
- Lore seemed to be immune
When Lore is speaking with the Crystalline Entity, he says:
LORE: Very good. You've understand [sic] perfectly so far. Next, I'll signal that I'm about to transport something out, at which time the deflector shields will turn off for a moment, and if you move in at that time
Unfortunately, Lore was cut off from finishing his sentence, but it certainly sounds as though he was about to say that "if you move in at that time, you can attack". Assuming this is correct, it seems that Lore was going to make the Enterprise-D vulnerable to attack from the Entity, but he was going to remain active without any apparent protection from the Entity. This suggests to me that Lore knew he would be safe from whatever the Crystalline Entity does to organic beings. Lore is physically identical to Data (as per 'Brothers'), so if the assumptions here are correct, Data would have been safe.
- Data was left outside and no harm came to him
Again in 'Datalore':
DATA: I have always felt that it was done hurriedly, but I know little more. Here, sir. This is where the cruiser's landing party found the signal device that had led them here. And they found me lying there, sir.
(He indicates a platform, obviously man-made, on a rock shelf)
TASHA: You were just lying out there in the open? No identity record, no instructions?
DATA: Only a layer of dust.
Data was obviously not harmed by the Entity's deadly methods, despite not having any protection. Later in the episode they claim that Data was safe from the Entity because he wasn't alive at the time, just like Lore, although I don't buy this based on my argument in point 1 above. Also, what difference would it make if Data were 'alive' or not? Data must have energy reserves. As Data wasn't very well understood even after 27 years by the time of 'The Measure of a Man', it's quite unlikely the Starfleet team who discovered him would have known how provide him with the power he needed. I expect that Data still had energy within his reserves and I highly doubt that such energy would have been converted to organic energy if he were active.